Electric Saturn

[ETSubmariner]

You know, I thought of a birth too. A large object in a current was my immediate thought, but let's see what the data says before giving birth to our little chicken.

[fosborn]

mathew »Could this be similar to forces that might be seen in a planet birthing event?

I'm thinking because its similar to Jupiter's storms, its probably not.

Jan-2011- Cassini-images
Implied circulation patterns for the massive Saturn storm as interpreted/speculated from the false color methane image composite taken by Cassini on Jan 12, 2011. (The circulation pattern is consistent with earlier images also taken by Cassini).
The main storm (white oval) is a large cell rotating clockwise in the northern hemisphere (thus anticyclonic high pressure outflow). Zonal flow coming in from the east is pulled around and into the storm center along the SE edge of the storm. Along the NE, the upper easterly belt winds pull and stretch this edge along the belt/zone margin. With anticyclonic eddies (looking like little tadpole shapes) on the eastern side. Along the SW and spotted westward, larger deep (blue) cyclonic vortices can be see. Directly W of the storm, presumably in the "wake region" high clouds with an overall anticyclconic nature can be seen peeling off towards the W.
Overall, this pattern is similar to the anticyclonic Great Red Spot in Jupiter's atmosphere.
North is at top in the images.
http://www.flickr.com/photos/31678681@N07/5353321193/

[fosborn]

New thermal images from ESO’s Very Large Telescope (VLT) and other ground-based telescopes show swirls of warmer air and cooler regions never seen before within Jupiter’s Great Red Spot. The images enable scientists to make the first detailed weather map of the inside of the giant storm system. One observation illustrated by this image is the correspondence between a warm core within an otherwise cold storm system and the reddest colour of the Great Red Spot
http://www.eso.org/public/images/eso1010a/

I cut and adjusted this image to highlight what I thought was a similar pattern in the Saturn Storm.

[StefanR]

Lightning Mixes a Dark and Stormy Brew at Saturn

The story of lightning in Saturn’s atmosphere has been building slowly. It first showed up years ago as electrostatic discharges lasting fractions of a second, heard as static and interference in the natural radio signals emitted by Saturn. The visible flash of lightning was harder to capture. Recently, Cassini scientists were excited to obtain long-sought photographic evidence of lightning in the atmosphere. Now, accumulating visible-light and infrared observations are providing fresh evidence that the dark clouds associated with Saturn’s thunderstorms contain dark substances such as soot and other carbon products that are forged when lightning strikes methane.
[...]


Lightning is thought to occur predominantly in regions of Saturn’s atmosphere where there is a strong up and down motion caused by moist cloud convection. Electrical charge may build up from friction between frozen and liquid particles that rise and fall in the convecting clouds.

The methane is cooked into carbon by lightning about 100 kilometers (62 miles) below the visible atmosphere. Other lightning-generated products become mixed with upper level condensates such as water and ammonium hydrosulfide. The resulting carbon-contaminated particles are icy, solid and unusually dark. The convection causes ammonia gas in the upper atmosphere above the lightning-generation level to rise and bright ammonia clouds become visible. This is followed by the appearance of the dark clouds that bring their cargo of carbon soot to the light of day to be viewed by Cassini.

To explain the dark clouds observed throughout the spectrum from the visible to near infrared observed by the visual and infrared mapping spectrometer (VIMS) onboard Cassini, VIMS team scientist Baines and chemist Delitsky analyzed results from a variety of laboratory spark experiments that tested how methane-laden chemistry like Saturn’s responds to the effect of lightning. They found that lightning discharges can reduce hydrocarbons such as methane to elemental carbon. Repeated electrostatic discharges into methane gas yield solid carbon, acetylene and other hydrocarbons. Temperatures inside a lightning strike on Saturn can reach over 30,000 Kelvin (53,540 degrees Fahrenheit), far hotter than needed to produce solid carbon from methane in Saturn’s atmosphere. That conversion can occur at temperatures as low as 2,000 Kelvin (3,140 degrees Fahrenheit).

The authors say that carbon fired deep inside Saturn may be the dominant darkener of the clouds found in and near Saturn’s thunderstorms. In addition, “lightning on Saturn will input large amounts of energy to a narrow column of atmosphere and generate products at high energies such as radicals and ions. After the column cools down, the new chemical species recombine and are frozen into a new chemical equilibrium which includes carbon soot but also incorporates a veritable soup of organic compounds and exotic species that include mixtures of sulfur, nitrogen, and phosphorus,” said Baines. Lightning, therefore, may kick-start a process that leads to increasingly complex chemistry in Saturn’s atmosphere.

The work directly addresses a Cassini mission objective to investigate the sources and nature of Saturn’s lightning and another to investigate the planet’s chemistry and dynamics, and is helping to characterize the dynamics that connect Saturn’s visible upper atmosphere with its deep interior. Ultimately, said Baines, “understanding more about lightning’s effects sheds light on the chemical processes at work in atmospheres that on some planetary bodies could lead to the origin of life.”

http://saturn.jpl.nasa.gov/news/cassini ... e20101006/

[StefanR]

Thunderstorms and lightning on Saturn

...imagine a huge thunderstorm with a diameter around 3000 km and lightning bolts whose radio signals are 10.000 times stronger compared to their terrestrial counterparts. These are the SEDs (Saturn Electrostatic Discharges) which are investigated in this project...

And in December 2010 a giant thunderstorm started in Saturn´s northern hemisphere with a latitudinal extension of 10,000 km (see below)!

Abstract (in English)

The purpose of this project is a successful continuation of the research activity on atmospheric electricity at the gas giant Saturn and its biggest moon Titan by analyzing data from the Cassini/RPWS (Radio and Plasma Wave Science) instrument. Radio waves from lightning discharges at Saturn (shortly named "SEDs" for Saturn Electrostatic Discharges) occur together with prominent cloud features imaged by Cassini/ISS (Imaging Science Subsystem) in Saturn's atmosphere. Many papers about SEDs have been published recently in refereed journals by the writer of this proposal. A new, not yet analyzed months-long lightning storm on Saturn has provided intriguing new data from which new findings can be expected. Combined observations of radio and optical emissions of Saturn lightning should lead to new clues about the morphology and frequency of occurrence of thunderstorms in Saturn's atmosphere. The SED related cloud features have diameters of several thousand kilometers, and they can even be imaged by amateur astronomers from Earth. A statistical analysis of the recent and further SED storms with respect to burst duration and rates, intensity, and frequency spectrum will be performed. Unique high temporal resolution observations of SEDs will be investigated in detail and compared to terrestrial intracloud strokes. SEDs act as a natural tool to probe Saturn's ionosphere, and the ionospheric peak electron density (an important parameter for scientists studying Saturn's ionosphere) will be determined from the SED low frequency cutoff. The SEDs can be detected even when the storm is "over the horizon" by as much as 45° when viewed from Cassini. This peculiar bending of radio waves by the planet's ionosphere needs to be studied in more detail and ray-tracing calculations are planned to be made. The first measurements of intense SEDs close to Saturn will allow us to evaluate if direction-finding is possible with short and bursty radio waves like SEDs. The polarization of SEDs will be studied in more detail for the new Cassini as well as the old Voyager SED observations, because the latter have never been analyzed systematically. The main target of this project is Saturn, but the search for lightning in the atmosphere of Saturn's enigmatic moon Titan will also be continued. Some results of the research on Saturn lightning can be used (1) to compare SED observations of RPWS with Earth-based SED observations and (2) to provide input for a micro- and macro-physical modeling of thunderclouds and lightning in Saturn's atmosphere.

http://www.saturnlightning.oeaw.ac.at/index.html

[StefanR]

Planetary atmospheric electricity
R. G. Harrison1, K. L. Aplin2, F. Leblanc3 and Y. Yair4
Abstract
Electrification is a fundamental property of planetary atmospheres, found widely in the solar system. It is most evident through lightning discharges, which can influence an atmosphere’s chemical composition, but electrification also affects the physical behaviour of aerosols and cloud droplets that determine an atmosphere’s radiative balance. In the terrestrial atmosphere, lightning has been implicated in the origin of life.

6. Conclusions
Atmospheric electricity can originate from many different causes, specific to the kind of atmosphere and the altitude above the planetary surface. In all gaseous planetary atmospheres charge is generated by cosmic rays or ultraviolet radiation, but also by friction, meteoric impacts, atmospheric circulation, cloud charging, volcanism, and dust. The presence of aerosols modifies the charging process, facilitating charge transfer between ions and aerosols. In other planetary environments, dust storms or impacts are responsible for charge production. Rings around planets can become charged both actively and passively – for instance in the presence of magnetic fields and plasma – with their dynamics being at least affected, if not completely determined, by the built up electric fields. In one or the other of these ways, charged atmospheric layers can be produced near most of the planets.
Information about planetary atmospheric electricity has been obtained by spacecraft observations, and the extraordinarily valuable measurements made in situ when spacecraft pass through a planetary atmosphere or even land instrumentation on the planet. On Earth, the violent discharge from large atmospheric electric fields is common and evident through lightning. On other planets, however most discharges manifest themselves in secondary effects, which, from lightning, is frequently the non-optical electromagnetic radiation that can leak from their atmospheres.
Consequently detailed study of terrestrial lightning remains very important, and therefore many of the articles in this book are in one or the other way concerned with lightning, its causes, mechanism, effects, intracloud lightning, cloud to ground lightning, and cloud to space lightning like TLEs, as well as the generation of electromagnetic radiation from Schumann resonances through the spectrum to X-rays, and TGFs. The latter two are signatures of high-energy particles generated in the lightning discharges; they may indicate planetary atmospheric electric fields which could present hazards for missions to the planets.
In this volume (Leblanc et al 2008), a detailed and up to date summary is given of the various problems in planetary atmospheric electricity outlined above. A feature is that it begins with an introductory overview section. These introductory overviews are intended to provide a brief, though fairly-founded, tutorial concerning the physics and chemistry of atmospheric electricity on Earth and the planets, the charging, ion production, current flow, current systems, electrical conductivities, and the observable processes involved into the quiet and the violent discharges. Based on these articles, the student and researcher of the many and various forms of atmospheric electricity should be prepared to deal with the more specialised papers that follow.

http://epubs.cclrc.ac.uk/bitstream/3026 ... eprint.pdf

full document can be had here:
http://www.ebooks-freedownload.net/tag/ ... lectricity

content index of document can be had here:
http://www.gbv.de/dms/goettingen/575188375.pdf

[flyingcloud]

not sure which thread to add this on to...

note the distinction and differences between radio emissions detected from both Saturn and Jupiter

Cassini finds Saturn sends mixed signals

http://www.physorg.com/news/2011-03-cassini-saturn.html

Recent data from NASA's Cassini spacecraft show that the variation in radio waves controlled by the planet's rotation is different in the northern and southern hemispheres. Moreover, the northern and southern rotational variations also appear to change with the Saturnian seasons, and the hemispheres have actually swapped rates. These two radio waves, converted to the human audio range, can be heard in a new video:

<embedded video in original article>

"These data just go to show how weird Saturn is," said Don Gurnett, Cassini's radio and plasma wave science instrument team lead and professor of physics at the University of Iowa, Iowa City. "We thought we understood these radio wave patterns at gas giants, since Jupiter was so straightforward. Without Cassini's long stay, scientists wouldn't have understood that the radio emissions from Saturn are so different."

Saturn emits radio waves known as Saturn Kilometric Radiation, or SKR for short. To Cassini, they sound a bit like bursts of a spinning air raid siren, since the radio waves vary with each rotation of the planet. This kind of radio wave pattern had been previously used at Jupiter to measure the planet's rotation rate, but at Saturn, as is the case with teenagers, the situation turned out to be much more complicated.

When NASA's Voyager spacecraft visited Saturn in the early 1980s, the radiation emissions indicated the length of Saturn's day was about 10.66 hours. But as its clocking continued by a flyby of the joint ESA-NASA Ulysses spacecraft and Cassini, the radio burst varied by seconds to minutes. A paper in Geophysical Research Letters in 2009 analyzing Cassini data showed that the Saturn Kilometric Radiation was not even a solo, but a duet, with two singers out of sync. Radio waves emanating from near the north pole had a period of around 10.6 hours; radio waves near the south pole had a period of around 10.8 hours.

A new paper led by Gurnett that was published in Geophysical Research Letters in December 2010 shows that, in recent Cassini data, the southern and northern SKR periods crossed over around March 2010, about seven months after equinox, when the sun shines directly over a planet's equator. The southern SKR period decreased from about 10.8 hours on Jan. 1, 2008 and crossed with the northern SKR period around March 1, 2010, at around 10.67 hours. The northern period increased from about 10.58 hours to that convergence point.

Seeing this kind of crossover led the Cassini scientists to go back into data from previous Saturnian visits. With a new eye, they saw that NASA's Voyager data taken in 1980, about a year after Saturn's 1979 equinox, showed different warbles from Saturn's northern and southern poles. They also saw a similar kind of effect in the Ulysses radio data between 1993 and 2000. The northern and southern periods detected by Ulysses converged and crossed over around August 1996, about nine months after the previous Saturnian equinox.

Cassini scientists don't think the differences in the radio wave periods had to do with hemispheres actually rotating at different rates, but more likely came from variations in high-altitude winds in the northern and southern hemispheres. Two other papers involving Cassini investigators were published in December, with results complementary to the radio and plasma wave science instrument -- one by Jon Nichols, University of Leicester, U.K., in the same issue of Geophysical Research Letters, and the other led by David Andrews, also of University of Leicester, in the Journal of Geophysical Research.

In the Nichols paper, data from the NASA/ESA Hubble Space Telescope showed the northern and southern auroras on Saturn wobbled back and forth in latitude in a pattern matching the radio wave variations, from January to March 2009, just before equinox. The radio signal and aurora data are complementary because they are both related to the behavior of the magnetic bubble around Saturn, known as the magnetosphere. The paper by Andrews, a Cassini magnetometer team associate, showed that from mid-2004 to mid-2009, Saturn's magnetic field over the two poles wobbled at the same separate periods as the radio waves and the aurora.

"The rain of electrons into the atmosphere that produces the auroras also produces the radio emissions and affects the magnetic field, so scientists think that all these variations we see are related to the sun's changing influence on the planet," said Stanley Cowley, a co-author on both papers, co-investigator on Cassini's magnetometer instrument, and professor at the University of Leicester.

As the sun continues to climb towards the north pole of Saturn, Gurnett's group has continued to see the crossover trend in radio signals through Jan. 1, 2011. The period of the southern radio signals continued to decrease to about 10.54 hours, while the period of the northern radio signals increased to 10.71 hours.

"These papers are important in helping to explain the complicated dance between the sun and Saturn's magnetic bubble, something normally invisible to the human eye and imperceptible to the human ear," said Marcia Burton, a Cassini fields and particles scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif., who was not involved in the work. "Cassini will continue to keep an eye on these changes."

[jjohnson]

This is an absorbing, intriguing finding, with a lot of data being taken and displayed, and corroborating (or at least somewhat correlated) findings in the UV aurora images and the magnetic field variations versus the seasonal aspect of Saturn's tilt. What was left unexplained (maybe it's obvious; I dont know) is the cause of this kilometric radio wave radiation?

Is it from a source near the poles, or from Saturn's ionosphere, like the whistler waves in our own ionosphere? Are there Alfvén waves in the plasma features around Saturn, with a radio frequency emission aspect? If the radio waves are kilometres in length (say from 1 to 10 km, as an example) then their frequencies would range from about 30 to 300 kHz. Other wavelengths and frequencies are possible as well. What mechanism on, in or in the vicinity of a gas giant planet accounts for this? It is way below the IR bands, and of course optical and UV. Synchrotron radiation can result in radio waves, as evidenced by active radio galaxies.

Anyone have any plausible explanations? NASA might know, and I've only seen the one video and article, and I'd be interested to see the source pinpointed and a mechanism proposed.

Jim

[StefanR]

Anyone have any plausible explanations? NASA might know, and I've only seen the one video and article, and I'd be interested to see the source pinpointed and a mechanism proposed.

This is from the Saturn Currently Stormy-thread:

Saturn’s radio broadcasters mapped in 3-D for the first time

Observations from NASA’s Cassini spacecraft have been used to build, for the first time, a 3-D picture of the sources of intense radio emissions in Saturn’s magnetic field, known as the Saturn Kilometric Radiation (SKR).

The results will be presented by Dr Baptiste Cecconi, of LESIA, Observatoire de Paris, at the European Planetary Science Congress on Tuesday 23rd September.

The SKR radio emissions are generated by high-energy electrons spiralling around magnetic field lines threaded through Saturn’s auroras. Previous Cassini observations have shown that the SKR is closely correlated with the intensity of Saturn’s UV aurora and the pressure of the solar wind.
The measurements were made using Cassini’s Radio and Plasma Wave Science (RPWS) experiment.
http://www.lesia.obspm.fr/~cecconi/file ... -movie.mp4
“The animation shows radio sources clustered around curving magnetic field lines. Because the radio signals are beamed out from the source in a cone-shape, we can only detect the sources as Cassini flies through the cone. When Cassini flies at high altitudes over the ring planes, we see the sources clearly clustered around one or two field lines. However, at low latitudes we get more refraction and so the sources appear to be scattered,” said Dr Cecconi.

The model found that the active magnetic field lines could be traced back to near-polar latitudes degrees in both the northern and southern hemisphere. This matches well with the location of Saturn’s UV aurora.

http://www.thunderbolts.info/wp/forum/phpB ... 365#p10365

So in a sense Flyingcloud's first question might be answered as well, this topic in that thread(???).

That was it, back to the studio, back to you Jim.

[mathew]

The latest from sky and telescope-
http://www.skyandtelescope.com/news/122329429.html

Back on December 5th, instruments aboard the Cassini spacecraft picked up radio and plasma-wave signals from a thunderstorm brewing in the planet's northern hemisphere. Only four days later, amateur astronomers in Japan spotted a new white atmospheric disturbance on the planet. It rapidly grew in an east-west direction from its vertex at 41° north latitude; within a month it was spanning 100° of longitude.

Since then amateur and professional scopes have been tracking the evolution of one of the largest storms ever reported on Saturn. Amateur stacked-video images and high-res pictures from Cassini, taken as recently as May 15th show white cloudy material wrapped around the northern hemisphere and spanning 30° of latitude.

Storms of this magnitude seem to occur once every Saturnian year (29.4 Earth years). During these events, gases from deep in the troposphere boil up into the lower stratosphere, causing large temperature changes that disrupt the subtle seasonal patterns on the planet and modify the chemistry of the upper atmosphere.



At center and on the right are thermal infrared images of Saturn from the mid-Infrared VISIR instrument at the European Southern Observatory's Very Large Telescope on Cerro Paranal, Chile. At left is an amateur visible-light image from Trevor Barry of Broken Hill, Australia. The images were obtained on Jan. 19, 2011, during the mature phase of the northern storm. The second image was taken at 18.7 microns, a wavelength that reveals heat coming from below and shadowy structures blocking it in Saturn's lower atmosphere. The third image, taken at 8.6 microns, is sensitive to higher altitudes in Saturn's normally peaceful stratosphere, where we see the unexpected beacons of infrared emission flanking the central cool region over the storm.
ESO/Univ. of Oxford/T. Barry

Such spectacular sights have been observed only five previous times since 1876. Until now they could be studied only in reflected sunlight. This time, however, not only do we have a spacecraft on the scene but astronomers on the ground can probe into the atmosphere at infrared wavelengths. Data from the Composite Infrared Spectrometer (CIRS) on Cassini, and thermal imaging from the Very Large Telescope (VLT) at Chile, have presented a team of scientists led by Leigh Fletcher (Oxford University) an unprecedented opportunity to quantify the changes in Saturn's stormy zones.

Two spectroscopic readings by Cassini — the first taken last October 22nd (well before the storm began) and the second on January 19th (45 days after its onset) — show a 7K to 9K drop in temperature. Thermal imaging shows that the cloud upheaval causes a darkened cold spot over the 3,000-mile-wide vortex and warms areas around the periphery . Over the storm's head, the airmass appears streaked with warm zones, dubbed stratospheric beacons.

As clouds continued to spew upward into the stratosphere, atmospheric layers were being displaced by about 20 ± 5 km over the heart of the storm, and 10 ± 4 km at its outer margins. These displacements churned the gases in the planet's atmosphere violently, causing, for instance, a depletion of acetylene (C2H2), a tracer for stratospheric motion, over the heart of the storm. Phosphine (PH3) showed a sudden enhancement of 20% in the stormy zones. Fletcher's team concludes, "The thermal contrasts resulting from the planetary-scale disturbance have a considerable affect on atmospheric motion."

While only the broadest picture is within the reach of amateur telescopes, it's exciting to peek beneath the blanket of stormy clouds on Saturn by way of images from more powerful instruments. And the show won't end soon — scientists believe that the uplift vortex will linger for a few years and continue to present a visible white spot long after most of the storm loses its steam.

[seasmith]

~
CYCLES COMPRESSING ?

Such storms, called "Great White Spots" due to their size and brightness, can be seen by Earthbound astronomers and occur on average every 30 years or so—approximately the length of a year on Saturn—but for some unknown reason this year's storm has appeared much earlier in the Saturnian spring than normal.

Instruments on board the Cassini probe detected bursts of radio waves generated by lightning flashes that, at their peak, occurred at least 10 times per second, an international team of researchers reports today in Nature.

http://news.sciencemag.org/sciencenow/2 ... b769cbe25e

[Komorikid]

This is the latest news on the new storm raging on Saturn.
I found this statement rather interesting -

The lightning is produced in water clouds, where falling rain and hail generate electricity.

I didn't know there was any liquid water or hail for that matter on Saturn.

See here

[StefanR]

A Major Variation on a Theme
Jul. 18, 2011
Stephen J. Edberg, Cassini science communication coordinator

Saturn has been recognized as a standout since Galileo first pointed his telescope at it. The beauty of the rings immediately draws attention. But Saturn’s invisible radiation belts are special too. Trapped in Saturn’s magnetosphere, the volume of space controlled by the planet’s magnetic field, the radiation belts are only present between zones limited by the orbital distances of the planet’s moons. The radiation disappears where the moons orbit because the particles collide with the moons. The radiation belts themselves should have disappeared long ago because the particles slowly spiral in towards the planet. But the belts are still there!

A recent paper in the Journal of Geophysical Research by Cassini scientists working with an energetic particle detector on the spacecraft’s magnetospheric imaging instrument provides an explanation: Saturn’s radiation belts are regularly replenished through the collision of galactic cosmic rays coming from outside the solar system with atoms in Saturn’s atmosphere and its rings. This phenomenon, known as cosmic ray albedo neutron decay, makes Saturn unique in the solar system and gives scientists a better understanding of the behavior of Saturn’s belts and their differences from Earth’s.

Saturn’s magnetosphere, like those of the other planets, contains radiation belts. These belts consist of ions (atoms that have lost one or more electrons, primarily by collisions or excitation by ultraviolet or x-ray radiation from the sun) and electrons that cycle back and forth along the field lines connecting the north and south magnetic poles. (The ions are mostly the nuclei – single protons – of hydrogen atoms.) After visits by NASA’s Pioneer 11 and Voyager spacecraft, scientists observed that Saturn’s rings and inner satellites absorb these ions and electrons as they orbit around Saturn, clearing zones in the radiation belts. How, then, do the belts maintain themselves over time when the particles slowly drift in, and therefore across the orbits of the satellites?

The graphic shows the donut-shaped structure of Saturn's ion radiation belts, constructed using data from Cassini's energetic charged particle detector known as the the low energy magnetospheric measurement system on Cassini’s magnetospheric imaging instrument. The intensity of the radiation belts is color-coded, with red and blue corresponding to high and low intensities, respectively. Saturn's ion radiation belts extend between the main rings and the orbit of Tethys. The large, inner Saturnian moons (Mimas, Enceladus, Tethys) absorb all high energy ions trapped in magnetic zones that are at the same distances as these satellites’ orbits. (Note how Mimas is particularly effective sweeping particles out of the zone of its orbit, marked by the blue arc in the cross-section.) This results in the formation of “shells” of radiation belts, which are also isolated from the particle populations residing beyond the orbit of Tethys. The intensities are for protons with energies between 12 and 59 million electron volts.
Image credit: NASA/JPL-Caltech/MPS/JHUAPL

Roussos and colleagues have found that the rain of local radiation on the moons is maintained by the combination of cosmic rays streaming in from our Milky Way galaxy and the nuclear physics of the cosmic ray collisions at Saturn. Although these kinds of cosmic ray collisions also occur at Earth (and possibly at Jupiter, Uranus and Neptune), the radiation belts of those planets may receive energetic particles through many different mechanisms. At Saturn, cosmic ray collisions appear to be the only process providing high-energy particles, making its radiation belts a unique, filtered reservoir of cosmic ray products from our galaxy.

These cosmic rays are ions themselves and must have enough energy, first, to get inside Saturn’s magnetosphere (which is a barrier to them). The higher the energy of the cosmic rays, the deeper they can penetrate in Saturn’s magnetosphere. Some of these cosmic rays may even make it in as close as the distance of the planet’s main rings or atmosphere. There the collision of a galactic cosmic ray with an atom can shatter the nucleus and release a high energy neutron, one of the constituents of the nucleus of an atom. The important physics in the process is that (1) neutrons are not affected by magnetic fields (so they can travel in any direction) and that (2) neutrons have a limited lifetime: they decay into a lower energy proton, electron, and an anti-neutrino. The proton (and electron) can then become constituents of Saturn’s radiation belts, replenishing particles lost to collisions with Saturn’s moons and rings.

This is a nice picture, but is it correct? Other researchers propose that the now-familiar process of coronal mass ejections from the sun could replenish the radiation belts. Roussos and colleagues point out that three intense solar events collided with Saturn’s magnetosphere and actually generated a temporary new radiation belt outside Tethys’ orbit, a moon orbiting at the outer edge of Saturn’s permanent ion radiation belts. However, no changes were detected in those inner radiation belts throughout this period. Tethys’ absorption of particles apparently isolates the inner radiation belts from events outside. Occasional injections of particles just don’t work to maintain or intensify the inner radiation belts.

Except for some simulations that match the particle energy spectrum observed, there was not observational support for either cosmic ray collisions or an internal acceleration process at work. The Cassini researchers took advantage of Cassini’s long stay studying the Saturn system and the fortuitous coincidence of the extended sunspot minimum just now ending to study any correlation between the behavior of the radiation belts’ intensities over time.

At first glance, cosmic ray collisions and a lack of sunspots would not seem to be related at all. But the galactic cosmic rays that are the engine of cosmic ray albedo neutron decay are modulated by the sun’s behavior. The cosmic rays need to have enough energy to enter the heliosphere (the sun’s version of a planet’s magnetosphere, but with more phenomena). The energy necessary to enter is lower when the sun is less active: this occurs when there are fewer sunspots and fewer solar flares and a weaker magnetic field carried by the solar wind (which is really more like a breeze during sunspot minimum). As a result, the number of galactic cosmic rays reaching Saturn’s magnetosphere is higher during sunspot minimum.

This effect occurs also at our planet and was first established by Scott Forbush in 1937. It is now recognized that there can be long-term Forbush effects tied to the sunspot cycle and short-term Forbush effects due to solar activity like coronal mass ejections. The researchers found evidence of both long- and short-term Forbush effects in their studies of the radiation belts.

In the paper led by Roussos, data from the low energy magnetospheric measurement system on Cassini’s magnetospheric imaging instrument (MIMI/LEMMS) showed that radiation belt intensity rose from the time Cassini arrived at Saturn (June 2004) through the first months of 2010 in step with the rise of the galactic cosmic ray intensity getting into the heliosphere. Then the sun started showing signs of renewed sunspot activity. This finding was in agreement with expectations for radiation belts generated by cosmic rays and represents long term Forbush effects in Saturn’s magnetosphere.

Short-term effects were less obvious but more intriguing. As sunspot activity was falling during the 15-month period spanning the last quarter of 2004 to the start of 2006, three “Solar Energetic Particle” (SEP) events were observed at Saturn. These are enhancements of cosmic rays, but their origin is the sun (solar cosmic rays). Solar energetic particle events can happen anytime but are more frequent when sunspot numbers are high.

Studies at Earth show that these solar cosmic rays supply other planets’ magnetospheres and their radiation belts with a significant number of high energy particles. This is what one would naturally expect to happen.This is not, however, the case for Saturn, in part because Tethys isolates the inner Saturnian belts from particles coming from outside. Solar cosmic ray events “carry” with them a stronger magnetic field, which excludes galactic cosmic rays from their volume. Galactic cosmic ray intensity is therefore reduced when solar cosmic ray events are moving across Saturn’s magnetosphere (these reductions are called “Forbush decreases”). Such Forbush decreases were identified in the MIMI/LEMMS dataset during the “active” 2004 to 2006 period.

Interestingly, during this same period increases in radiation belt intensity were lower (or even absent) compared to the increasing intensities seen between 2006 and 2010. This indicates that Forbush decreases may actually limit the high-energy particle content of Saturn’s radiation belts and even cause a reduction in their intensity (since fewer galactic cosmic rays could enter and have collisions). If this finding is verified by subsequent observations, it may mean that Saturn’s radiation belts are the only ones known to us whose intensity is reduced, instead of being enhanced, after a solar energetic particle event.

Still, some mysteries remain. While cosmic ray collisions may explain the presence of high-energy ions at Saturn, the radiation belts also contain lower energy products. The research team also found that, if the data were separated into lower and higher energy particles in the radiation belts, both behaved the same way. This suggests that both have the same origin or that both might be tied together by some cause-effect relationship, but scientists have not yet understood where the low-energy population comes from or how the high- and low-energy products are related.

http://saturn.jpl.nasa.gov/news/cassini ... e20110714/

[seasmith]

Still, some mysteries remain. While cosmic ray collisions may explain the presence of high-energy ions at Saturn, the radiation belts also contain lower energy products. The research team also found that, if the data were separated into lower and higher energy particles in the radiation belts, both behaved the same way. This suggests that both have the same origin or that both might be tied together by some cause-effect relationship, but scientists have not yet understood where the low-energy population comes from or how the high- and low-energy products are related.

Would be interesting to see an animated graphic of Saturn's location in solar orbit- correlated to cycles of galactic/solar gamma irradiations and radiation belt intensities ...

http://saturn.jpl.nasa.gov/news/cassini ... e20110714/

[MosaicDave]

This has to be one of the most fascinating locations that I have ever seen, from which to take a photo of a planet:

http://www.scientificcomputing.com/news ... 91411.aspx

I wonder what interesting things can be seen in this image, from an EU perspective?

What do you make of the hazy luminous streaky bands at weird angles above and below the planet and rings? I wonder if these represent real physical things being imaged, or if they're just some weird type of lens flare...

--dc